Steam is commonly used in industry as a medium for transporting heat energy from a central location via suitable pipelines to its point of use. The heat energy conveyed in the steam may be released at a suitable point by causing a phase change of the steam from as to liquid. The phase change releases latent heat from the steam and causes liquid condensate to form.
The condensate and any trapped air needs to be removed from the pipelines, because its presence can cause corrosion and reduce the system's heat transfer efficiency. However, any device for removing condensate also needs to prevent steam from escaping, as this also reduces the system's heat transfer efficiency and may waste the energy transported by that steam.
GB 2 304 300 describes an example of a continuous flow fixed orifice (CFFO) steam trap. In such devices, an orifice is located in the flow path of steam travelling through a pipeline. The size of the orifice is chosen carefully such that the condensate, which flows through it much more slowly than the steam, acts to block the steam from passing. The diameter of the hole can be very small, e.g. about 0.4 mm in some cases.
Another type of orifice steam trap, known as a venturi orifice trap, has the orifice formed as part of a venturi, i.e. a conduit having a constricted passage therein, the constricted passage having a smaller cross-sectional area then the remainder of the conduit. In venturi orifice traps, the orifice is provided in the constricted part of the venturi, whereby condensate passing through the orifice enters channel that opens out (e.g. flares outwardly) in the direction of flow. The orifice may thus be provided at the upstream end of a channel having an increasing cross-sectional area. The pressure drop after the orifice causes flash steam to form from the saturated condensate, whereby the venturi restriction is understood to regulate the condensate flow. Such channels have been used in venturi orifice steam trap demonstrated to operate over varying condensate flows without passing live steam. U.S. Pat. No. 4,745,943 describes such an arrangement. Venturi orifice traps are used in systems that exhibit varying loads, e.g. in which the amount of condensate and hence the flow rate of condensate through the trap varies over time (e.g. depending on the level of demand experiences by the heat transfer system).
It is important that the small hole in an orifice trap should not be blocked by debris or particulate matter carried in the condensate or condensable gas. Sufficient flow rate must be maintained through the hole to prevent build-up of condensate in the trap and thereby minimise corrosion. The debris itself may also cause corrosion. GB 2 478 536 describes a steam trap incorporating a perforated barrier upstream of the orifice to filter debris and prevent blocking of the orifice.